Today, wireless communication network/systems such as 3rd Generation Partner Project (3GPP) Long Term Evolution (LTE) have been widely deployed to provide various types of communication content, such as voice content and data content. It may be expected the mobile wideband traffic will increase dramatically, which raises higher demand on coverage and capacity of the system.
Currently, operators address this problem by increasing the density of deployment, typically, by deploying more base stations to cover broader areas and access more users. However, it is usually expensive for the operators to deploy new base stations. Base stations are connected to core network generally via wired connections such as optical fiber and cable. The construction of fiber and cable is expensive and time consuming, and the maintenance is also expensive. In some specific circumstance, construction of fiber and cable is not feasible due to geographic limitation. Although it is proposed to use microwave to connect the base stations and the core network, the property of microwave communication requires a line-of-sight transmission, which may seldom satisfied in scenarios like dense city.
As an alternative, the concept of relay is proposed in LTE-advanced (LTE-A) by 3GPP and standardized in 3GPP TS 36.216 V10.2.0 Physical layer for relaying operation (Release 10), March 2011 and 3GPP TS 36.300 V10.3.0, E-UTRAN Overall description; Stage 2 (Release 10), March 2011. FIG. 1 shows a schematic view of an existing LTE relay solution. As shown in FIG. 1 an intermediate node 120 referred to as relay node is added between a base station 120 and a User Equipment (UE). The base station 110 is herein referred to as donor base station or macro base station. The link between the donor base station 110 and the UE 130 is divided into two segments, where the link between the donor base station 110 and the relay node 120 is referred to as backhaul link (Un), and the link between the relay node 120 and the UE 130 is referred to as access link (Uu). The main task of the relay node 120 is to forward data between the donor base station 110 and the UE 130. In particular, in downlink, all the traffic which was destined from the base station 110 to the UE 130 is first transmitted to the relay node 120 and then forwarded to the UE 130 by the relay node 120; in uplink, all the traffic which was destined from the UE 130 to the base station 110 is first transmitted to the relay node 120 and then forwarded to the base station 110 by the relay node 120. Generally, the relay node 120 does not simply amplify and forward signals at the Layer 1 (PHY layer), but may implement protocols at the Layer 2 (including Medium Access Control (MAC)/Radio Link Control (RRC)) or even higher layer. The relay node 120 may be divided into two parts (not shown) in functionality. One part is referred to as UE part, which looks as if the UE from the perspective of the donor base station 110; the other part is referred to as base station part, which looks as if the base station from the perspective of the UE 130. In the downlink, the UE part of the relay node receives radio frequency (RE) signal and converts it to baseband signal. The base station part of the relay node processes the baseband signal, converts it to RF signal and transmits it to the UE 130. In the uplink, the base station part of the relay node receives RE signal from the UE 130 and converts it to baseband signal. The UE part of the relay node processes the baseband signal, converts it to RF signal and transmits it to the base station 110. Since the backhaul link Un between the relay node 120 and donor base station 110 could be LTE air interface instead of wired or microwave interface, the deployment of relay node may be economical and easy-to-implement as compared with deploying new base stations.
However, most of current research works on relay are focused on standardization, few on the implementation of relay product, especially the relay product for LTE TDD. Vendors may have to spend a lot of money and labor to develop and test relay products. In addition, operators may have difficulty in deployment and maintenance of new relay products, e.g. they need to do a considerable amount work on field debugging and personnel training.